If still dissatisfied, not only does the operator lose their repeat custom but that person will relish in relating all the gory details to friends and family. As much as Brits avoid confrontation, they love a good moan.
Most operators I’ve spoken to about this would rather deal with things there and then. It affords them the best opportunity to smooth things over and erases the laborious process of responding to comment cards, TripAdvisor etc.
The problem is how to get people to come clean at the time. A respected food blogger told me his reason for not doing so is that, as something has already gone wrong, he’d prefer to go home and write an unfavourable review rather than ruin the night further.
He has a point, but that attitude could be altered if the customer was able to complain safe in the knowledge that it wouldn’t escalate into an argument, but be dealt with appropriately and smoothed over with some sort of goodwill gesture.